1. Field of the Invention
The present invention relates to a GaN-series light emitting diode with high light efficiency and its manufacturing method, especially to a GaN-series light emitting diode and its manufacturing method, in which the strain of epitaxy layer is controlled during the process of epitaxial growth to form a p-type semiconductor layer with surface textured structure to interrupt the optical waveguide effect.
2. Description of the Related Art
A sapphire substrate grown GaN-series light emitting diode, according to prior art, shown in FIG. 1 has a so-called conventional structure, consisting of a GaN buffer layer 2, a n-type GaN ohmic contact 3, a InGaN light emitting layer 4, a p-type AlGaN cladding layer 5 and a p-type GaN ohmic contact layer 6 epitaxially grown sequentially on the sapphire substrate 1. A semi-transparent metal conductive layer 7 is formed on the p-type GaN ohmic contact layer 6, a p-side electrode 8 is formed on the semi-transparent metal conductive layer 7, and a n-side electrode 9 is formed on the GaN ohmic contact 3. Because of the refraction index distribution of said GaN epitaxial structure (n=2.4), sapphire substrate (n=1.77) and resin capping materials for packaging (n=1.5), only 25% of the light emitting from the light emitting layer can be output directly and not be reflected by the interface. 75% of the light are limited by the optical waveguide structure composed of the sapphire substrate and resin capping materials for packaging where light reabsorption takes place due to multiple interface reflection, thus preventing effective extraction of the generated light for utilization. The light extracting efficiency of said light emitting diode is limited by the absorption of the light at the semi-transparent metal conductive layer and reabsorption at the internal epitaxial structure.
A method of interrupting the optical waveguide effect is presented in U.S. Pat. No. 6,091,085 which improves the extracting efficiency of the LED by forming a rough texturing on the surface of sapphire substrate, and then growing multi-layer epitaxial structure of GaN-series of light emitting diode. Alternatively, the epitaxial structure of GaN-series of light emitting diode is grown directly on the sapphire substrate, and then tunnel channels are formed on the surface of the epitaxial structure. The tunnel channels extend in the direction of the sapphire substrate and contain the materials whose refraction index is smaller than the one (n=2.4) of multi-layer GaN epitaxial structure.
However, because the first method requires mechanical polishing or chemical etching to produce the rough texturing, it may cause the non-uniformity of surface roughness on the sapphire substrate, which may affect the conditions of the subsequent epitaxial structure, thus reducing the manufacturing yield. While the second method increases the complexity of fabrication and manufacturing cost due to fabrication of tunnel channels and embedded materials.
U.S. Pat. No. 6,495,862 discloses GaN-series of light emitting diode having convex contoured surface to reduce the reflection of the light emitted from the light emitting layer by semi-transparent metal conductive layer and the interface of resin capping for packaging and to increase the external quantum efficiency. However, making the surface with cylindrical or semi-circle convex texture is complex and expensive.
U.S. Pat. No. 6,531,710 discloses a method in which the optical waveguide effect is interrupted and flexural distortion generated from stress is reduced as the result of the epitaxy process which utilizes an epitaxially gown internal layer of AlN having reticulate linear structure located between the light emitting layer and sapphire substrate to interrupt the optical waveguide effect and to increase external quantum efficiency. This structure may be formed of a metal reflection layer on the AlN internal layer to reflect the light from the light emitting layer towards the sapphire substrate to increase external quantum efficiency. The method includes injecting ammonia gas (NH3) and trimethyalumium (TMA) into reaction chamber and controlling the flow of ammonia gas, while the internal layer of AlN is grown by utilizing the metal organic chemical vapor deposition (MOCVD), to achieve control of the reticulate linear shape, followed by the growth of multi-layer epitaxial structure. However, this method may easily cause the hexagonal shaped pits, as presented in the thesis (APL 71, (9), Sep. 1(1997), P. 1204). If the pits extend to the surface of p-type ohmic contact layer from the internal layer of AlN, metal atoms diffusion into the light emitting layer may happen which can disrupt the characteristics of light emitting diode device and shorten the operation life of the device when the consecutive semi-transparent conductive metal layer or metal electrode are formed.
According to the thesis of J. L. Rouviere et al (Journal of Nitride-Semiconductor-Research, Vol. 1, (1996) Art. 33), three types of surfaces may be formed (hexagonal pyramid shaped roughness, flat, and granular roughness) as the result of different conditions of epitaxial growth, of the thin film of GaN on the sapphire substrate by using MOCVD. The experiment proves that the shape of the surface is determined by the polarization direction and migration rate of surface atoms. When the mechanism of surface growth is mainly controlled by the N-polarity, the surface is rough; when the mechanism of surface growth is mainly controlled by the Ga-polarity, the surface is flat; and when the surface of GaN thin film is flat, the possibility of formation of hexagonal shaped pits is substantially reduced.
Therefore, providing a novel high light efficiency of GaN-series of light emitting diode and the manufacturing method thereof can eliminate the drawbacks of the prior arts, such as mechanical polishing or chemical etching to interrupt the optical waveguide effect. Based on the inventor's experience in research and development, as well as in sales of the related products for many years, the inventor proposes herein a method to overcome the aforementioned problems for improving a high light efficiency of GaN-series of light emitting diode and the manufacturing method thereof.